Ball bearings of the type shown in FIG. 1, a kind of rolling bearings, are widely used for bearing various rotating parts. A ball bearing is composed of a concentrically assembled set of inner race 2 and outer race 4, inner race 2 forming inner raceway track 1 (one track) on its outer circumference, and outer race 4 forming outer raceway track 3 (the other track) on its inner circumference, with a plurality of balls 5 rotatably arranged as rolling members between inner raceway track 1 and outer raceway track 3. Each of annular seal plates 6 is fitted, at one end thereof, to the inner circumferential surface of outer race 4 at each end of outer race 4 in its width direction so that the grease applied around balls 5 or dust generated may not leak outside or dust suspending outside may not enter in the vicinities of the balls.
Plural balls 5 are rotatably retained by cage 7 shown in FIGS. 2 and 3. Cage 7 is made of an integrally injection-molded synthetic resin. Cage 7 integrally consists of annular main body 8 having a plurality of retaining parts 9 on one side thereof. Each retaining part 9 is formed by a pair of elastic lugs 10 spaced from each other. The opposing faces of a pair of elastic lugs 10 have a concave curved surface thereby to make a spherical form.
Each of balls 5 is pressed into the space between a pair of elastic lugs 10 while elastically widening the interval between the pair of elastic lugs 10 and is thereby rotatably retained by each retaining part 9.
Cage 7 is generally made of a metal or a synthetic resin which can be injection molded, such as a polyamide resin, a polyacetal resin or a polybutylene terephthalate resin, or a synthetic resin composition comprising such a synthetic resin and glass fiber, carbon fiber, organic fiber, etc.
Since cage 7 only functions to retain balls 5 lubrication of a ball bearing is achieved by using a lubricating oil or a semi-solid lubricant, such as grease. However, where, in particular, a semi-solid lubricant such as grease is used, resistance to stirring of the lubricant increases the torque required for rotating a shaft borne by the bearing. Besides, the torque fluctuation increases with rotation.
Such a high torque, wide torque fluctuation, and noise of the bearing give rise to great problems on practical use in the case of small diameter ball bearings having an inner diameter of not more than 6 mm, such as those incorporated into HDD, VTR, DAT or LBP.
In order to solve these problems, a ball bearing in which cage 7 itself has a lubricating function has been proposed. For example, JP-A-U-4-133023 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application") discloses a rolling bearing cage in which the U-shaped space of a section in the axial direction of a column-shaped part is filled with a solid lubricant (plastic grease).
JP-A-61-6429 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a bearing cage made of a polyamide-imide resin having open cells at a void volume of 7 to 17% impregnated with a completely fluorinated oil.
JP-A-1-93623 discloses a cage which is produced by injection molding an oil-containing plastic composition comprising a nylon plastic (6,6-nylon) and a binder containing about 10% mineral oil to obtain a cage having an oil content of 10% and then immersing the resulting cage in a mineral oil at 120.degree. C. for 7 days to increase its oil content to 30%.
These conventional cages having a lubricating function involve the following disadvantages, however. That is, production of the cage of JP-A-U-4-133023 requires labor in filling plastic grease into the U-shaped space of the column part. This cannot be done through a single injection shot, resulting in increase of cost.
Additionally, when the plastic grease in the U-shaped space is solidified by heat treatment followed by allowing to cool, the plastic grease undergoes shrinkage to a higher degree than with the frame, i.e., the U-shaped part, tending to form a gap between the solidified plastic grease and the frame. The gap is broadened as the grease oozes out on use of the bearing. It follows that the plastic grease moves in the U-shaped space, making abnormal noise.
Production of the cage disclosed in JP-A-61-6429 requires much labor for providing open cells. Moreover, the cage tends to have low mechanical strength due to much void. Further, since a polyamide-imide resin, which is a hydrocarbon-based compound, has insufficient capability of retaining a completely fluorinated oil, i.e., a fluorine-containing compound, the oil oozes out relatively soon, failing to maintain a lubricating function for a prolonged period of time in a stable manner. Furthermore, since a fluorinated oil is inferior to a hydrocarbon oil in lubricating action, the bearing tends to have reduced acoustic characteristics.
Production of the cage of JP-A-1-93623 ; involves immersion of a molded article in a lubricating oil at a high temperature (120.degree. to 130.degree. C.) for a long time (about 7 days) in order to increase the oil content. The lubricating oil and the plastic are apt to be deteriorated during the immersion. The molded article undergoes considerable dimensional change on oil immersion, and the change is instable and varies widely among lots. Bearings using such a cage show large scatter in performance quality.
Moreover, since a large proportion of the lubricating oil is retained in the shallow portion in the vicinity of the surface, oozing of the lubricating oil occurs relatively in the early stage of use, resulting in a failure of maintaining stable lubrication for a long period of time.
Thus, the conventional cages having a lubricating function have various drawbacks or problems and do not withstand practical use sufficiently.